The present invention relates to solvent stable composite membranes which have particular application to the processing of liquid streams and in particular the purification and concentration of solutes, especially but not exclusively biologically active materials, which solutes are dissolved in organic solvents including mixtures of organic solvents as well as mixtures of organic solvents with water.
Composite membranes are known for performing separations of species from gaseous mixtures, vapors and their solutions in water or organic solvents.
Separation of the various components is usually based on imparting to the membrane a selection mechanism which enhances preferential adsorption or dissolution of certain selected species in the membrane phase, followed by its preferential transport across the membrane from the feed side to the permeate side. The transport of a selected species of solvent across a membrane is by means of a driving force, in particular an electrochemical potential, which is established across the membrane. Such a driving force can in general take the form of electrical potential, pressure difference, concentration differences and/or other driving forces either singly or in combination.
The present invention utilizes primarily pressure differences established across a membrane, in particular pressure driven processes such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis, but does not exclude the utilization of other driving forces such as concentration gradients and electrical driving forces. In the processing of liquid streams performed by the membranes of the invention, and in particular the separation of dissolved components present in such streams, it is preferred to apply superatmospheric pressure, thereby leading to transport of organic solvent and some solute, while retaining other dissolved solutes.
Streams which can be treated by means of solvent stable membranes of the present invention are:
(i) Lubricating oils, which are in particular low MW components having a MW cut off in the range of 300-2000 Daltons, and which are dissolved in strong organic solvents such as NMP, phenol, MEK, MIBK, toluene and their mixtures. Their separation requires availability of solvent stable membranes which will be stable in solvents such as those specified, and which will retain the dissolved low MW oils to a sufficient degree, e.g. between 70-95%.
(ii) Catalysts dissolved in organic solvents. Several catalysts comprising metal organic complexes are in commercial use for performing catalytically enhanced polymerization reactions in organic media. These catalysts are very expensive and there is great interest in recovering them from reaction mixtures. The molecular weight of the catalyst may vary from 200-300 and up to 2000-3000 Daltons.
(iii) Low MW oligomers in paint wastes dissolved in strong organic solvents such as MEK, butyl acetate and/or other strong solvents singly or in admixture.
(iv) Biologically active materials such as antibiotics and peptides, which are frequently prepared and purified in organic solvent media or mixtures of such media with water. The measures necessary to concentrate and purify such materials are often slow and costly, and moreover must be used with extreme care in order to prevent decomposition and loss of activity of such materials in the course of processing. The use of high boiling solvents such as dimethylformamide (DMF), dimethylacetamide (DMAC), dimethylsulfoxide (DMSO) and N-methylpyrrolidone (NMP) presents particular difficulties in this respect.